1. Field of the Invention
The present invention relates to a cylinder head for an internal combustion engine, and, more particularly, relates to the structure of a lower end of an intake port formed in the cylinder head.
2. Description of Related Art
In an internal combustion engine having two intake ports per cylinder, as illustrated in FIG. 4, one intake port 21 generates a swirl flow 23 in a cylinder 24 and the other intake port 22 causes a part of intake air to flow into the cylinder 24 in a direction 29 opposite to the swirl flow 23. This inhibits the generation of a strong swirl flow in the cylinder 24.
To weaken the swirl-opposing flow from intake port 22 to thereby improve a swirl ratio, Japanese Patent Publication No. SHO 56-148616 discloses an intake valve 26 provided with a shroud 25, as shown in FIG. 5.
The intake valve 26 is inclined from a direction perpendicular to a lower surface 30 of the cylinder head 27, as shown in FIG. 6. As a result, a dead volume 28 (a triangular portion (in cross-section) surrounded by lines A'B', B'C', and C'A') is defined between an extension of the lower surface 30 of the cylinder head 27 and a lower surface 26a of the intake valve 26.
However, the internal combustion engine having a plurality of intake ports and using the inclined intake valve having a shroud has the following problems.
First, by providing a shroud, the number of constituent parts increases. Further, to prevent the intake valve from rotating about its axis, resulting in dislocation of the shroud from its intended position, it is necessary to provide an additional valve rotation preventing device. Both of these factors increase the cost of manufacture.
Second, the dead volume caused because of inclination of the intake valve lowers the compression ratio of the engine and thereby reduces the engine power.